Heinrich
Lenz stated a law to determine the exact polarity of an induced voltage. In
doing that, Lenz reasoned that the process of induction must confirm to the
well-known principle in physics that states that reaction is equal and opposite
to action. Of several ways of stating Lenz’s law, we shall use the following
form;
Lenz stated a law to determine the exact polarity of an induced voltage. In
doing that, Lenz reasoned that the process of induction must confirm to the
well-known principle in physics that states that reaction is equal and opposite
to action. Of several ways of stating Lenz’s law, we shall use the following
form;
Lenz’s
Law: The polarity of the induced voltage must be such that the current
resulting from it develop a flux which tends to oppose any change in the
original flux.
Law: The polarity of the induced voltage must be such that the current
resulting from it develop a flux which tends to oppose any change in the
original flux.
Lenz’s
law can be demonstrated using figure 1 below.
law can be demonstrated using figure 1 below.
When
we first close the switch in the mutual induction demonstration of figure 1a,
the primary flux will increase from zero. Since a resistor is connected across
the secondary winding, according to Lenz’s law. The flux produced by this
secondary current must try to oppose the increase in primary flux. Applying the
Right Hand Rule to the primary winding, we find that the primary flux will have
a clockwise direction around the core. To oppose an increase in this clockwise
primary flux, the flux produced by the secondary current must have an
anti-clockwise direction.
we first close the switch in the mutual induction demonstration of figure 1a,
the primary flux will increase from zero. Since a resistor is connected across
the secondary winding, according to Lenz’s law. The flux produced by this
secondary current must try to oppose the increase in primary flux. Applying the
Right Hand Rule to the primary winding, we find that the primary flux will have
a clockwise direction around the core. To oppose an increase in this clockwise
primary flux, the flux produced by the secondary current must have an
anti-clockwise direction.
Again
applying the Right Hand Rule, this time to the secondary winding, we can
establish the direction in which the secondary current must flow through the
resistor, and thus determine the polarity of the induced voltage in terms of
the polarity of the voltage drop across the resistor.
applying the Right Hand Rule, this time to the secondary winding, we can
establish the direction in which the secondary current must flow through the
resistor, and thus determine the polarity of the induced voltage in terms of
the polarity of the voltage drop across the resistor.
When
we open the switch, as in figure 1b above, the primary current stops flowing
and the primary flux collapses. According to Faraday’s Law, this reduction in
flux will induce a voltage into the secondary, and according to Lenz’s law, the
secondary current must try to oppose the collapse of the primary flux. To help sustain the primary flux, which has a clockwise direction in the core, the
secondary current must also produce a clockwise flux direction. Applying the
Right Hand Rule, we obtain the polarity shown in figure 1b. The polarity of the
induce voltage when the primary current is decreasing is, therefore, opposite
to that induced when the primary current is increasing.
we open the switch, as in figure 1b above, the primary current stops flowing
and the primary flux collapses. According to Faraday’s Law, this reduction in
flux will induce a voltage into the secondary, and according to Lenz’s law, the
secondary current must try to oppose the collapse of the primary flux. To help sustain the primary flux, which has a clockwise direction in the core, the
secondary current must also produce a clockwise flux direction. Applying the
Right Hand Rule, we obtain the polarity shown in figure 1b. The polarity of the
induce voltage when the primary current is decreasing is, therefore, opposite
to that induced when the primary current is increasing.
We
can also use Lenz’s law to determine the direction of induced voltage of a
current carrying conductor in a magnetic fields separately, the field of a
stationary magnet has the form as shown in figure 2a below;
can also use Lenz’s law to determine the direction of induced voltage of a
current carrying conductor in a magnetic fields separately, the field of a
stationary magnet has the form as shown in figure 2a below;
If we assume at the moment that the conventional
current direction in the conductor is out of the page, the magnetic field
around the conductor has the form as shown in figure 2b above. Before we place
the conductor between the magnetic poles in the sketch and superimpose the two
magnetic fields, we must recall that magnetic lines of force can never
intersect. Therefore, the two composite magnetic field pattern, as shown in
figure 3 below.
current direction in the conductor is out of the page, the magnetic field
around the conductor has the form as shown in figure 2b above. Before we place
the conductor between the magnetic poles in the sketch and superimpose the two
magnetic fields, we must recall that magnetic lines of force can never
intersect. Therefore, the two composite magnetic field pattern, as shown in
figure 3 below.
Above the conductor (figure 2b)
the direction of the magnetic lines of force is opposite to that of the
permanent magnetic field (figure 2a). Consequently, there is a cancelling
effect which bends the resultant magnetic field away from the conductor, as
shown in figure 3 above. Below the conductor, the direction of its magnetic
field is the same as that of the permanent magnetic field. As a result, the
flux density is increased below the conductor in the composite flux pattern in
figure 3.
the direction of the magnetic lines of force is opposite to that of the
permanent magnetic field (figure 2a). Consequently, there is a cancelling
effect which bends the resultant magnetic field away from the conductor, as
shown in figure 3 above. Below the conductor, the direction of its magnetic
field is the same as that of the permanent magnetic field. As a result, the
flux density is increased below the conductor in the composite flux pattern in
figure 3.
A notable characteristic of
magnetic lines of force is that they tend to become as short as possible and
tend to repel one another. Therefore, the lines of force in the composite
magnetic field of figure 3 will attempt to straighten out and to space
themselves uniformly, in order to regain the flux pattern of figure 2a. To
accomplish this, the magnetic field will attempt to force the current-carrying
conductor to move upward. According to the principles stated in Lenz’s Law, the
force will oppose the actual motion of the conductor through the stationary
field. Consequently, when the electric conductor moves downward through the
stationary magnetic field, the voltage induced into the conductor has polarity
such as to cause the current in the close loop to have a direction out of the
page.
magnetic lines of force is that they tend to become as short as possible and
tend to repel one another. Therefore, the lines of force in the composite
magnetic field of figure 3 will attempt to straighten out and to space
themselves uniformly, in order to regain the flux pattern of figure 2a. To
accomplish this, the magnetic field will attempt to force the current-carrying
conductor to move upward. According to the principles stated in Lenz’s Law, the
force will oppose the actual motion of the conductor through the stationary
field. Consequently, when the electric conductor moves downward through the
stationary magnetic field, the voltage induced into the conductor has polarity
such as to cause the current in the close loop to have a direction out of the
page.
